1. Field of the Invention
The present invention relates to a circuit breaker that can be applied in a motor circuit for protecting a motor from overload and short-circuit, and in detail, to an adjustment dial for setting a steady state current value of a thermally actuated overload tripping device mounted on such a circuit breaker.
2. Background Art
It is known to use a type of circuit breaker commonly referred to as a “manual motor starter” for a switching device employed in a protecting and controlling circuit of a motor. The manual motor starter (abbreviated as “MMS”) is a circuit breaker compactly integrating the functions of a circuit breaker and a thermal relay (see JP-A-2003-100195, for example). Such a circuit breaker is ordinarily further combined with an electromagnetic contactor so as to carry out short-circuit and overload protection and operational control of a motor. An outline of the arrangement of the circuit breaker (MMS) is shown in each of FIGS. 4A, 4B and 5. FIG. 4A is a plan view showing a circuit breaker (MMS) with a case cover thereon, FIG. 4B is a plan view showing the inner mechanism of the circuit breaker (MMS) shown in FIG. 4A with the case cover removed, and FIG. 5 is a perspective view showing a unit assembly of a thermally actuated overload tripping device shown in FIG. 4B. Firstly, in FIGS. 4A and 4B, reference numerals 1 and 1a denote a main body case and case cover, respectively of a circuit breaker having a molded resin case. Reference numerals 2 and 3 respectively denote power source side terminals and load side terminals of the main body case 1. Reference numeral 4 denotes a switching mechanism carrying out a switching operation of a contact section of a main circuit. The switching operation of the switching mechanism 4 is performed by a lever 5 (a rocker system). Reference numeral 6 denotes an electromagnetic overload tripping device detecting a short-circuit current of the main circuit to instantaneously trip the circuit breaker, and reference numeral 7 denotes a thermally actuated overload tripping device detecting overload and open phase conditions to trip the circuit breaker. The contact section of the main circuit, though not shown, is incorporated in the main body case 1 on the bottom side thereof. Here, the thermally actuated overload tripping device 7, as shown in detail in FIG. 5, is a unitary assembly that combines a main bimetallic element 8 as a thermally actuated element corresponding to each phase in the main circuit, a differential shifter mechanism 9 including a differential lever linked to an operational end of the main bimetallic element 8, a push shifter and a pull shifter, a compensating bimetallic element 10 for ambient temperature compensation that also serves as a release lever linked to a latch catcher of the switching mechanism 4, with one end thereof made opposed to an output end 9a of the differential lever of the differential shifter mechanism 9, and an adjustment dial 11 for setting the steady state current value of the thermally actuated overload tripping device 7 to a capacity (rated current value) of a motor. In addition, for each phase, reference numeral 8a denotes a heater for heating the main bimetallic element 8.
The compensating bimetallic element 10 is held by a compensating bimetallic element holder 12a, which is pivotally held by a dial holder 12. The adjustment dial 11 is held by the dial holder 12 and is positioned so that the head of the adjustment dial 11 is aligned with an opening in the case cover 1a of the main body case 1 (see FIG. 4A) with a dial cam 11e, formed on the shaft of the adjustment dial 11, made to butt against the butting face (not shown) of the compensating bimetallic element holder 12a. FIG. 6A is a perspective view showing an adjustment dial mounted on the circuit breaker shown in FIGS. 4A and 4B, and FIG. 6B is a plan view showing an example of setting the setting pointer mark on the adjustment dial at a steady state current value in the circuit breaker. As shown in FIGS. 6A and 6B, the adjustment dial 11 has an arrow-shaped setting pointer mark 11a on the upper face of the head thereof. The setting pointer mark 11a is formed for setting a steady state current value with the setting pointer mark 11a to be set at the steady state current value on a current value scale 13 shown along a periphery of an opening in the case cover 1a. The setting pointer mark 11a is preferably formed as a slot into which the tip of a slotted screwdriver is inserted so as to turn the adjustment dial 11. A numeric character presented on the current value scale 13 represents the rated current value (A) of a motor. The setting range of the rated current value in the example shown in FIG. 6B is from 0.16 A to 0.25 A. The rated current value of the motor to be used is within this range in the example, and the setting is carried out so that the setting pointer mark 11a on the adjustment dial 11 is set at the rated current value. The operation and the function of the circuit breaker (MMS) is described in detail in the JP-A-2003-100195. Here, however, an explanation will be made relating to the thermally actuated overload tripping device 7. Specifically, when the circuit breaker is in actual use, the bending of the bimetallic element 8 heated by a current flow in a main circuit transfers the displacement due to the bending to the compensating bimetallic element 10 through the differential shifter mechanism 9. An overload state of the main circuit, that is, an increase in the current value in the main circuit larger than the steady state current value set beforehand by the adjustment dial 11, increases the displacement due to the bending of the main bimetallic element 8. The increased displacement due to this bending causes the output end 9a of the differential shifter mechanism 9 to push the compensating bimetallic element 10 to release the engagement between a latch and a latch catcher in the switching mechanism 4. This causes the switching mechanism 4 to be tripped to open the contact section in the main circuit.
The adjustment dial 11 is provided to set the steady state current value to the rated current value of a motor. The setting of the setting pointer mark 11a of the adjustment dial 11 shown in FIG. 6B at the position on the current value scale 13 corresponding to the rated current value of the motor, by turning the adjustment dial 11, causes the compensating bimetallic element holder 12a to turn on the dial holder 12 through the medium of the dial cam 11e of the adjustment dial 11. This displaces one end of the compensating bimetallic element 10 to cause play between the compensating bimetallic element 10 and the output end 9a of the differential shifter mechanism 9 to vary to be adjusted in correspondence with the set current value.
When the rated current adjustable circuit breaker (MMS) is used in a panel such as a control panel, the overload protection characteristic of the circuit breaker varies depending on how it is arranged. More specifically, the amount of bending of the main bimetallic element 8, as a thermally actuated element of the thermally actuated overload tripping device 7 contained in the main body case 1 of the circuit breaker, depends on the temperature of the bimetallic element. More specifically, the amount of bending depends on the temperature of the bimetallic element raised by applying an electric current to the bimetallic element. The temperature of the bimetallic element at this time varies according to ambient temperature conditions and, besides this, also according to the heat dissipation characteristic of the main body case 1 of the circuit breaker for dissipating the heat into the surroundings therefrom. The following is a comparison of the case of using one circuit breaker as a single item with the case of using a plurality of circuit breakers laterally arranged in line with adjacent ones in close contact with each other without providing any spacing. In the latter case, the heat dissipation from the main body case 1 of each circuit breaker is prevented by main body cases of the adjacent circuit breakers on the right and left sides lowering the heat dissipation characteristic of the main body case 1. This causes generated heat to be stored in the main body case 1 to excessively raise the temperature of the main bimetallic element 8, which results in a large amount of bending of the main bimetallic element 8. In such a state, even though the current value of the main circuit (the load current value of the motor) is equal to or less than the steady state current value set beforehand with the adjustment dial 11, there is a possibility of causing the thermally actuated overload tripping device 7 to be operated to accidentally bring the main circuit of the circuit breaker to an opened state, which causes an emergency stop in operation of the motor, to making adequate operational control impossible.
The circuit breaker as a product is provided so that steady state current values shown on the current value scale 13 are determined on the basis of the operating arrangement for a single item and an ambient temperature of 20° C. Thus, in the instruction manual accompanying a product delivered from the manufacturer, a caution is given such as “provide sufficient spacing between adjacent circuit breakers when mounting a plurality of circuit breakers in the same panel.” Nevertheless, in some cases in which mounting of additional circuit breakers is carried out by the user, limited space in a control panel may make it necessary to mount a plurality of circuit breakers on a mounting rail in the panel arranged in line with adjacent ones in close contact with each other and little spacing provided in between. The use of the circuit breakers in such an arrangement introduces a possibility of causing malfunction of the thermally actuated overload tripping device to accidentally bring the main circuit of the circuit breaker to an opened state even though the load current value of the main circuit is equal to or less than the steady state current value set beforehand. As was explained in relation to FIG. 5, the related thermally actuated overload tripping device 7 is provided with the compensating bimetallic element 10 to reduce the influence of variations in ambient temperature (standard temperature: 20° C.) on the operating characteristics of the circuit breaker. The compensating bimetallic element 10, however, has a limitation in its functionality. More particularly, in the arrangement of rated current adjustable circuit breakers (MMSs), in each of which the steady state current value is set at the rated current value of a motor by means of the adjustment dial, in line in a panel with adjacent ones in close contact with each other, there is a possibility that the compensating bimetallic element 10 cannot by itself prevent the malfunction of the thermally actuated overload tripping device 7 to accidentally bring the main circuit of the circuit breaker to an opened state.
In one method that can be used to counter this problem, an adequate correction value for a steady state current is calculated on the basis of a relation between the temperature rise of the main bimetallic element and its operation characteristic, which relation varies depending on whether a single item of the circuit breaker is used or a plurality of the circuit breakers are arranged with adjacent ones in close contact with each other, and the adjustment dial is reset to the corrected value. This measure, however, is troublesome and additionally necessitates expert knowledge. Thus, such a measure cannot be commonly applied. The invention was made in view of the foregoing with an object of providing a circuit breaker which is capable of adequately setting the steady state current value of a thermally actuated overload tripping device by a simple adjustment dial operation of only setting a setting pointer mark at the steady state current value on a current value scale corresponding to the capacity of a motor, whether one circuit breaker is used as a single item or a plurality of circuit breakers are arranged in line with adjacent ones in close contact with each other.